The modern papermaker employs a highly sophisticated papermaking machine which is in essence a device for removing water from the paper furnish. The water is removed sequentially in three stages or sections of the machine. In the first or forming section, the furnish is deposited on a moving forming fabric and water drained through the fabric to leave a paper sheet or web having a solids content of about 18 to 25 percent by weight. The formed web is carried into a press fabric section and passed through one or more nip presses on a moving press fabric to remove sufficient water to form a sheet having a solids content of about 36 to 50 percent by weight. This sheet is then transferred to the dryer section of the papermaking machine where dryer fabrics hold the paper sheet against hot, steam-heated dryer cylinders to obtain about 92 to 96 percent solids content. The papermaking fabrics employed on the papermaking machine must perform a diverse range of functions, according to the position on the machine, i.e., forming, press or dryer section.
Forming fabrics used in the papermaking process are a kind of papermaking fabric which are used in the forming section of a papermaking machine. Forming fabrics are generally constructed of synthetic yarns joined together, ordinarily by weaving, in a fabric construction that is characterized by a high degree of open spaces between the intersecting yarns. Forming fabrics must maintain a high degree of openness to insure that they permit removal of water from the fiber slurry deposited thereon.
Since water removal capability is a critical function of the forming fabric, it is necessary to insure that the fabric retains a high degree of openness over its lifetime.
However, the degree of openness of a fabric is continually reduced during its life. In addition to the fiber slurry, paper pulp ordinarily contains additives such as filler clay, pitch, and polymeric materials that clog the open spaces of the fabric. The use of recycled fibers has introduced considerable amounts of contaminants in the form of inks, adhesives, tars, and polymeric materials, which also clog the open spaces of the fabric.
In addition, forming fabric designs now include multilayer fabrics that are more susceptible to contamination problems.
Accordingly, it is desirable to provide a fabric which exhibits an improved degree of contamination resistance. One proposed prior art solution is the use of contamination resistant yarns in the construction of the fabric. This has not proved to be wholly satisfactory since the contamination resistance provided by such yarns is short-lived and/or ineffective. Another proposed solution calls for coating or treating paper making fabrics in order to improve their resistance to contaminants. Again, this method is not wholly successful because the contamination resistance provided by the coating is short-lived and/or ineffective.
One problem inherent to coatings or treatments is that coatings per se are known to reduce the permeability of a fabric, an undesired result that inhibits water removal capabilities, the primary function of a forming fabric. It is therefore important that any coating applied to a forming fabric reduce permeability as little as possible.
It is also desirable to improve shear stability of a papermachine clothing. Shear stability is the degree to which the filaments of a fabric can shift before opposing filaments lock them into place.
U.S. Pat. No. 5,207,873 discloses a coating for papermaking fabrics in order to render them contaminant proof by increasing the anti-stick properties of the fabric. The fabric is treated with 1% solids solution containing poly(tetrafluoroethylene), urethane copolymers, polyacrylamide, acrylic copolymer, methylene bisacrylamide, polyaziridine cross linker, methyl pyrolidone, ammonium persulfate solution, sodium metabisulfate solution, urea peroxide solution, and silver nitrate solution, the components being present in the coating in the amounts and concentrations set forth in the disclosure. This coating has not proved to be totally effective and/or permanent on paper machine clothings.